1. Field of the Invention
The invention relates to the field of image reproduction with photosensitive elements, particularly those comprising polymeric layers, which are peeled apart in the process of forming reverse images. Such elements are particularly useful for image reproduction for color proofing and related photomechanical uses, e.g., as lithographic transparencies, photomasks, photoresists, etc.
2. Description of the Prior Art
The prior art, e.g., Burg et al., U.S. Pat. No. 3,060,024 and U.S. Pat. No. 3,060,025; Celeste et al., U.S. Pat. No. 3,607,264; and Phlipot, U.S. Pat. No. 3,236,647, describes photopolymer elements and processes of use. Uncolored, photopolymerizable layers are selectively colored by applying colorants to imagewise exposed, clear photopolymerizable layers, so that the pigment adheres selectively to the unexposed areas. Chu and Cohen, U.S. Pat. No. 3,649,268 describes a process in which an element having a removable support and a photohardenable layer is (1) laminated to a receptor layer, and (2) imagewise exposed through the support to actinic radiation which selectively raises the stick temperature of those areas receiving the radiation, then (3) the support is stripped from the layers, and (4) the outer surface of the layer is treated (e.g., dusted) with a material, usually a colored pigment which adheres only to the unexposed areas of the layer thereby producing a positive image. By repeating the laminating, exposing with color separation records, stripping and treating steps in sequence, a multicolor image can be obtained. However, the process has the limitation of producing only a positive-working or duplicate image (i.e., the pigment image is a duplicate of the image on the transparency used for exposure) and is therefore restricted in use, particularly where it is desired to produce a negative-working or reverse image (i.e., an image which is complementary to the image on the transparency used for exposure).
Since the elements of the prior art generally produce toned images by adherence of toner to unexposed areas of a photopolymerizable layer, they produce duplicate images. An element is needed which will produce reverse toned images by a dry process for color proofs and photomasks, and in other areas where photopolymer systems are useful, e.g., color design work using custom colors. The elements of the prior art are also limited in the materials that can be used to provide a proper adhesion relationship, and peel apart elements of the prior art may have pinholes in the image due to improper adhesion. Adhesive values of prior art peel apart elements are typically below 20 g/inch with differences between exposed and unexposed areas being a fraction of this value. Such peel apart elements are characterized in the publication "Image Formation by Photoadhesion" by Woodruff et al., Photographic Science and Engineering, Vol. 11, No. 2, pp. 93-97, March-April 1967. Because of their limited tonal range and image fidelity such peel apart systems are restricted to less demanding uses, eg, engineering drawings etc.